A record-breaking 4,855 people queued for hours in the rain to be tested to see if they were a match to help save the life of Oscar Saxelby-Lee.
DKMS, a charity that tests stem cell swabs, said its previous record for the highest number of people to take part in a registration event was 2,200 people.
Pitmaston Primary School, in Worcester, opened its doors for a donor search for Oscar at the weekend.
His parents Olivia Saxelby and Jamie Lee, of Worcester, launched an appeal to find a match after Oscar was diagnosed with T-cell acute lymphoblastic leukaemia.
They aimed to get as many people as possible to sign up to a blood stem cell donor register as part of their campaign "Hand in Hand for Oscar".
Doctors have said they may have just three months to find a match and save the child's life.
Ms Saxelby-Lee, 23, said: "We felt like we could not see light at the end of the tunnel, but when looking at Oscar's cheeky smile, bravery and determination, we managed to pull our strength together again.
“From that moment of fear and confusion, we as a family became stronger than ever. Oscar reminded us how to fight again and just how courageous he is.
“Not once has he shown weakness, nor has he ceased to amaze us throughout the most difficult times and that to us is a true warrior.
“Oscar is a fun, loving, energetic five-year-old boy who deserves to live to the full alongside the other troopers fighting such horrific diseases.
“Not only does he need to enjoy a normal life a child should live, he now needs someone else to save him.”
Oscar's teacher Sarah Keating said: “I've been teaching for 20 years and I've never had a child go through something like this.
“You hear about children getting cancer and you think, 'That's dreadful', then you move on. In this case we haven't moved on, we will fight this.”
Join our new commenting forum
Join thought-provoking conversations, follow other Independent readers and see their replies